1. Field of the Invention
This invention relates to a double superheterodyne receiver, and particularly relates to such a receiver in which the first and second local oscillator circuits thereof are formed as phase-locked loop circuits.
2. Description of the Prior Art
Double superheterodyne receivers, which are also variously known as double-conversion receivers or triple-detector receivers, are commonly used in, for example, UHF communications. Such receivers provide high gain without instability, good suppression of image frequencies, and high-adjacent channel selectivity.
In these receivers, an incoming RF carrier frequency is mixed with a first local oscillator signal to produce a first IF signal, the latter is mixed with a second local oscillator signal to provide a second, or final IF signal, and such signal is provided to a detector to detect the information carried on the second IF carrier.
Various arrangements of double superheterodyne receivers have been proposed for receiving several AM frequency bands including, for example, a low-frequency or long-wave band, a broadcast frequency or medium-wave band, and several high-frequency or short-wave bands. The short-wave bands normally include a plurality of discrete bands, for example, a twenty-meter band, a sixteen-meter band, and a ten-meter band. Such a receiver is designed to attempt to receive as many frequencies as possible between about 100 KHz and several dozens of MHz.
In the previously proposed double superheterodyne receiver, a first main tuning dial is used to control the first local oscillator for coarse tuning, while a second or auxiliry tuning dial is provided for fine tuning to a specific frequency. The main tuning dial and the auxiliary dial each have a respective dial display. The dial display for the main tuning dial is usually divided into long-wave, medium-wave, and several short-wave portions, while the dial display for the auxiliary tuning dial has divisions corresponding to small frequency differences, for example, divisions of 1 KHz.
The receiver is tuned by rotating a main tuning knob until a pointer on the main tuning dial display is roughly aligned wth indicia indicating the approximate frequency desired. Then, a knob on the auxiliary tuning dial is adjusted until a fine-tuning indicator registers that the receiver has been tuned the desired frequency.
While such previously proposed double superheterodyne receivers can tune accurately to a desired frequency, they are rather bulky and are somewhat cumbersome to operate.
With a previously proposed receiver, it is necessary to adjust both the main tuning dial and the auxiliary tuning dial to establish the received frequency every time a new frequency is selected.
Furthermore, because each of the tuning knobs must have a dial display associated with it, the dial display requires a rather large area in the radio receiver and makes it difficult to construct the receiver as a compact unit.
Additionally, for receiving single side-band (SSB) transmissions on a previously proposed double superheterodyne receiver, it is necessary to provide a change-over switch to set the receiver to an SSB mode, and to provide a third tuning dial and an associated third rotary knob so that the receiver can be finely tuned with an accuracy of about 100 Hz. This, of course, requires still another display device, but graduated in units of about 100 Hz. Thus, if an SSB transmission is to be received, operation of the change-over switch and the three rotary knobs make control of the receiver somewhat troublesome.